The present invention relates to piston and connecting rod retention and/or connection in a positive displacement, reciprocating piston-in-cylinder devicexe2x80x94whether a pump or prime mover, such as an internal combustion engine.
U.S. Pat. No. 5,146,883, DE-A-4,308,751, U.S. Pat. No. 5,305,684, U.S. Pat. No. 4,858,566, U.S. Pat. No. 4,459,900, U.S. Pat. No. 3,173,344, U.S. Pat. No. 2,819,936, UK-A-293,506 and CIMAC Helsinki 1981 Conference Paper Dl 09, variously disclose xe2x80x9cspherically-jointedxe2x80x9d piston and connecting rod assemblies with a multi-part retaining ring of split, or distinct, elements or parts, but none with a subtended arc angle of more than 180 degrees.
The connecting rod assembly itself is also generally in two parts. One part comprises a small end, shank, and upper big end bearing housing. Another part comprises a connecting rod end cap, which forms the lower portion of a circular bearing housing for the big end bearing. Suitable fasteners are installed to hold these elements together.
The joint between piston and connecting rod small end includes a bearing (surface) allowing relative (rotational) movement. While the piston is confined by its contact with cylinder wallsxe2x80x94to at least a linear reciprocating motion within the cylinderxe2x80x94the nature and degree of freedom of relative movement admitted between piston and connecting rod (small end) joint, reflects the joint configuration. The joint bearing surface must be adapted accordingly.
Thus, a (part-)spherical joint allows both (connecting rod) articulation or tilting, and rotation of the piston about its axis. This contrasts with a conventional cylindrical journal bearing of a so-called xe2x80x9cgudgeonxe2x80x9d or xe2x80x9cwristxe2x80x9d pin joint, which allows tilting, but not rotation.
Generally, piston rotation tends to adopt a sporadic form, spreading wear around the cylinder wall circumference, rather than a continuous rotation, which could engender an adverse (localized, e.g. annular) wear mode. More specifically, in a spherical joint, complementary opposed bearing surfaces are formed on the small end of a connecting rod; the underside of a piston body; and the upper side of a piston retaining ring. There is necessarily a small clearance between the various bearing surfaces of the piston and connecting rod assembly, to allow relative rotation, but to minimize relative axial movement.
Principal advantages of a substantially spherical, or part-spherical, joint bearing contact surface, compared to a more conventional (gudgeon or wrist) pinned joint, include: (a) piston expansion is symmetrical about its longitudinal (reciprocating) axis, affording manufacturing simplification, and smaller running clearances; (b) piston skirt wear is spread more evenly around the entire piston (skirt) circumference, promoting longer piston service life; and (c) the bearing area available for carrying principal compressive load can be increased thus either reducing bearing loading, or increasing the load carrying capacity.
Hitherto, as with the particular art identified, (part-) spherical piston-connecting rod bearings have generally used a split (or multi-part) retaining ring, to allow installation within the piston and around the connecting rod. Commonly, a further retaining ring is employed to hold this split ring to the piston body as, for example, demonstrated in the disclosures of SAE Paper 960055, DE-A-4,308751, U.S. Pat. No. 2,819,936. In one variant in SAE Paper 960055, an entire (part-) spherical bearing surface is formed in the body of the piston, with no separate retaining ring. The machining and assembly complexities attendant these solutions are unattractive for mass production.
According to one aspect of the present invention, a piston assembly comprises a piston fitted with a connecting rod coupled thereto by a (pivot or swivel) joint, itself retained by a unitary (retaining or retention) ring. Another aspect of the invention provides a unitary (retaining or retention) ring, for such a piston assembly. The retaining ring could be configured as either a partly, or completely, closed loop.
The retaining ring is installed upon, or within, a piston to lie generally transversely of the piston axis. The retaining ring cross-section is uniform, or (periodically) varied throughout its circumference, for example, providing a series of spaced bearing contact regions or xe2x80x9clandsxe2x80x9d, protruding or upstanding from a lesser ring cross-section. A certain symmetry of form is desirable. An undulating, or corrugated, profile can be employed.
The retaining section could change orientation throughout its circumference, e.g. by twisting, provided again a symmetry of collective or cumulative bearing surfaces were preserved.
The overall retaining ring profile or contour can be flat, or at least before installation xe2x80x9ccantedxe2x80x9d or periodically xe2x80x9cwavyxe2x80x9d, with corrugated forms providing contact lands. Thus, for example, in a part-closed retaining ring (but distinct from the minor, arcuate ring segments of the art), opposite ends could lie in different planes. A helical, or part-helical ring would be a case in pointxe2x80x94again desirably providing a substantially symmetrical overall bearing geometry, if not of the whole ring, then localized contact lands. The helix could be compacted into a flat, or at least flatter form, upon installation, providing a tight sprung fit, without the need for supplementary circlips or other fasteners.
Generally, a unitary retaining ring configuration represents a simplification in construction, manufacture and assemblyxe2x80x94over the known multi-part ring art identified.
Preferably, the internal profile of such a unitary retaining ring, and the (complementary) external profile of the associated connecting rod, allow the ring to pass over the rod, (even at its point of greatest cross-section, that is usually its big end, with the big end bearing cap removed.
The retaining ring may be configured as a continuous closed loop, with an asymmetric internal aperture profile to complement or fit around the connecting rod cross-section. Alternatively, the retaining ring may be only partially closed, that is, of less than 360 degrees circumferential span, for example, configured as a form of horseshoe. Such a partially-closed retaining ring need not pass over the connecting rod big end, but rather may be fitted laterally onto the connecting rod shank. In either case, the component count is less than for a split retaining ring assembly of the known art identified. Moreover, the surfaces to be machined are readily accessible and of relatively simple form.
Overall symmetry of piston and bearing configuration provide stable expansion characteristics, in turn promoting: a low engine oil consumption; and a reduced leakage (xe2x80x9cblow-byxe2x80x9d) of working fluid. These benefits tend to prevail throughout a long useful working life, since the ring grooves will not suffer the asymmetric distortions that occur with more conventional pinned joints.
Some means of positive mechanical entrainment between the retaining ring and piston (internal) wall is desirably employed for a secure inter-connection. To this end, conveniently, the retaining ring has an external thread, to mate with a complimentary threaded internal bore in the piston wall.
Torque tool locating recesses, or modest protruding lugs, may be incorporated in the ring body, to facilitate tightening of the threaded interconnection with the piston. Alternatively, a circlip may be fitted beneath, or into a circumferential wall slot within, the retaining ring, in order to locate in a groove or ledge in the internal piston wall.
In another embodiment, several circumferentially-spaced, such as longitudinally-directed threaded fasteners, may be fitted to pass through the piston retaining ring into the body of the piston (crown). Radial fasteners can be an alternative or supplementary approach.
A piston assembly with connecting rod retention according to the invention is compatible with engines needing high cylinder pressure capability, in order to minimize emissions and fuel consumption. This compatibility arises largely through the increased bearing area, but also by improved stress distribution and minimal shape distortion.
Hitherto known pistons capable of withstanding the stresses produced in high pressure engines generally employ steel for the main structure. Some examples would be steel-crowned xe2x80x9carticulatedxe2x80x9d pistons; single-piece cast, or fabricated, steel pistons; and steel-crowned, composite pistons. With the adoption of such robust steel pistons, very high bearing pressures are encountered, calling for advanced bearing materials or treatments which tends to increase their cost. In contrast, a piston with connecting rod retention of the present invention can be made at much lower cost (than steel), preferably in aluminum alloy, although cast iron would also be well-suited.
The attendant large bearing area is particularly suitable for arduous duty, such as is experienced with two-stroke diesel engines. Operationally, in a two-stroke combustion cycle, there is no load reversal and so the small end bearing design is critical. Additionally, since, in a two-stroke cycle engine, the load is always compressive in the connecting rod, the big end bearing itself may not need a (substantial confinement or retention) closure cap. Similarly, in a two-stroke cycle engine, the main load-bearing part of a connecting rod big end circumference need only embrace an arc some 120 degrees or less.
Other preferred features of a piston assembly adopting piston and connecting rod retention according to the invention include: wear-resistant ring carrier material (e.g. Ni-resist) ; reduced top-land height; integral coolant gallery; and ceramic fiber reinforcement. Generally, the piston assembly will be of aluminum alloy, with a steel connecting rod, although any suitable material combination may be used.